1. Field of the Invention
The field of art to which this invention pertains is hydrocarbon coking operations and in particular a process and apparatus adding refinery or petroleum sludge and oily wastes to a coking zone.
2. General Background
Coking operations in most modern refineries produce solid coke, vapor, and liquid products from heavy residual oil feedstocks which are fed to the coking process. The coking process can be either a delayed coking operation or a fluidized coking operation.
In fluid coking, a feedstock contacts a fluidized bed of coke particles maintained at a sufficiently high temperature to affect conversion of the feed to lighter liquid and vapor materials which are recovered from the fluidized bed of coke. Some of the coke particles formed in this operation are passed into a separate gasifier vessel where a portion of the coke particles are burned to produce additional heat. Heat is thus recycled back into the fluid bed of coke particles in the reaction section through these higher temperature coke particles which help maintain the desired process operations.
In the delayed coking process, delayed coking drums are used wherein a heavy residual oil is heated in a furnace and passed through a transfer line into a coking drum. In the coking drum, which is typically an elongated, cylindrical, vertically positioned vessel with an outwardly convex top and a downwardly converging frusto conical bottom, the residual feedstock is thermally decomposed with time into solid coke and vapor materials. The vapor materials formed during the coking reaction are recovered from the delayed coking drum and a solid coke material is left behind.
The vapor products are removed from the top of the coke drum through a coke drum outlet and passed through a coke drum overhead line which is connected to a fractionator, often called a combination tower. In the combination tower, gasesous and liquid products are recovered for further use in the refinery.
After a period of time, the feed to the coke drum is stopped and routed to another drum, and the coke laden drum is then purged of vapors, cooled, and opened so that the solid coke inside the drum can removed.
In operating a coking process, with the exception of needle coke production, the refiner generally aims to minimize coke production and maximize liquid products, since the latter are more easily converted into gasoline or other materials having higher economic values than the solid coke material.
Sludge production from a typical refinery or petrochemical plant can come from many sources including API separator bottoms, slop oil emulsions, storage tank bottoms, sludge from heat exchangers, oily waste, MEA reclaimer sludges, and other waste materials produced in the plant. The typical refinery sludge will contain solids, which may be organic, inorganic or combinations of both, along with oil, liquid and aqueous materials. Sometimes the sludge contains predominantly liquid materials and can be in the form of an emulsion.
In most refinery or petrochemical operations the sludge is often sent to a separator for gross removal of water and hydrocarbons after which the water and concentrated hydrocarbons and solids can be individually treated by landfarming or further biological or other known waste treatment means.
The refining industry has attempted to use various processes of adding sludge to a coking zone for sludge disposal.
U.S. Pat. No. 4,552,649 (U.S. Class 208/127) describes an improved fluid coking process where an aqueous sludge which comprises organic waste material is added to a quench elutriator to cool the coke in the elutriator and convert at least a portion of the organic waste to vaporous compounds which can be recycled to the fluid coking heating zone to increase the temperature of the fluid coke particles therein.
In the delayed coking process, sludges have been disposed of in various manners.
In U.S. Pat. No. 3,917,564 (U.S. Class 208/131), sludges or other organic by-products are added to a delayed coking drum during a water quenching step after feed to the coke drum has been stopped and the coke drum has been steamed to remove hydrocarbon vapors. The quenching step cools the hot coke within the coke drum to a temperature that allows the coke to be safely removed from the coking drum when it is opened to the atmosphere.
The sludge is added along with quench water and contacts the solid coke in the coke drum during the quench step at conditions which allow the vaporization of the water and some hydrocarbons contained in the sludge. Other organic and solid components of the sludge are left behind through deposition on the coke and removed from the coke drum as part of the solid coke product.
U.S. Pat. No. 4,666,585 (U.S. Class 208/131) relates to the disposal of petroleum sludge in a delayed coking process by adding the sludge to the coker feedstock and subjecting the mixture to delayed coking conditions.
German Patent DE 372606A1 relates to the disposal of petroleum sludge by adding sludge to the coke drum of a delayed coking process. The patent does not teach or disclose industry problems associated with solids entrainment and carryover or sludge injector reliability. U.S. Pat. No. 2,043,646 (U.S. Class 202/16) discloses a process for the conversion of acid sludge into sulfur dioxide, hydrocarbon and coke in a two-step procedure comprising passing sludge into a kiln to produce semi-coke and then passing the semi-coke into a coke drum for conversion into coke product.
U.S. Pat. No. 1,973,913 (U.S. Class 202/37) discloses a process where coke, which has been removed from a coking oven or coking drum, is quenched with polluted wastewater which contains tar acids. After quenching the tar acids can remain on the coke and the aqueous materials associated with these acids is vaporized.
U.S. Pat. No. 4,874,505 (U.S. Class 208/131) discloses a process where sludges are segregated according to water content. Sludges with a high water content are used as quench stream during the quenching phase of the coking cycle. Low water sludges are injected into the coke drum feed during the coking phase of the coking cycle.
U.S. Pat. No. 4,404,092 (U.S. Class 208/131) discloses a process for increasing the liquid yield of a delayed coking process by controlling the temperature of the vaporous space above the mass of coke in the coke drum by injecting a quenching liquid into the vapor phase within the delayed coking drum. The patent teaches that large amounts of liquid should be added to the vapor space within a delayed coking drum (about 9 percent by weight of the feed).
U.S. Pat. No. 2,093,588 (U.S. Class 196/61) discloses a process for delayed coking in which liquid materials, such as hydrocarbons or water, are passed into the vapor portion of a delayed coking zone. This patent teaches a process very similar, if not identical to, that disclosed in U.S. Pat. No. 4,404,092 described above.
In some of the alternative processes described above, certain disadvantages are present.
In cases where the sludge is added to the coke drum during the quenching cycle, the temperature of the solid coke which the sludge contacts may not be high enough decompose the sludge to coke and hydrocarbon vapors. While vaporization of the water contained in the sludge by the hot coke might occur, a concern exists that there may not be sufficient conversion or vaporization of the hydrocarbon component of the sludge. If the sludge contains toxic substances, they might not be converted to more acceptable and safer components.
In operations that inject sludge directly into the coke drum during a coking cycle, operational reliability problems can occur. The sludge injection apparatus is subjected to high temperatures and materials that can cause it to plug or be rendered inoperable. The sludge itself, once injected into the drum, can become entrained in the upflowing coke drum vapor stream, carried out of the coke drum, and deposited in downstream equipment such as the exit vapor piping or combination tower. Solids carryover in vapor piping can cause line fouling and excessive pressure drop that may exceed drum relief valve pressure. Solids carryover to the combination tower can plug the tower bottom outlet or plug the tower trays. Either item can require or cause a unit shutdown.
One of the other prior art processes entails the injection of sludge into the combination tower or directly into the coker hydrocarbon feed line. If the sludge is added to the combination tower or to any of the coker feed materials which pass through the coke heater furnace, there is a potential for fouling or coking of the furnace tubes or coker transfer lines because the sludge contains solids and often highly cokable hydrocarbon materials or materials that catalyze coking reactions. Additionally, in such instances, it is advisable to remove substantially all of the water from the sludge prior to injection into a high temperature hydrocarbon environment and consequently, additional processing equipment for this dewatering step is required. This is especially true if injection is into the furnace transfer line or into coke drum below the upper section.
A third alternative is injection of sludge into the coker blowdown system. In such a process, sludge is injected into the upper portion of the oil scrubber in the coker blowdown system and contacted with hot coke drum vapors during coke drum blow down, which can last a few hours a day. Water and light oils are vaporized and go overhead. Solids and heavy oil go out the bottom, and the heavy slop is fed to the coker combination tower and eventually passes through the coker feed furnace, transfer line and into the coke drum. This particular processing sequence also presents potential problems concerning the fouling of furnace tubes and feed or transfer lines to the coker.
It is an object of the present invention to convert a sludge which contains water and organics, and in other cases, water, liquid organics and solid organic or inorganic materials in a coking zone to recover useful and valuable products from the sludge.
It is an additional object of the invention to reduce the export of waste materials from a refinery or chemical plant by converting generally available sludges in a coking zone.
It is an additional object of the present invention to increase the yield of saleable or valuable products from sludge materials by processing them in a coking zone to convert at least a portion of the wastewater sludge to coke or liquid materials, which can be recovered from the coking process.
It is an additional object of the present invention to meet the above objectives without reducing liquid yields of the hydrocarbon feedstocks passed into the coking zone, and, additionally, without overloading downstream processing equipment with large volumes of aqueous vapor which need to be condensed.
It is an additional object of the present invention to perform the above objects without substantially reducing the partial pressure of hydrocarbons within the vapor phase within the coking zone.
It is an additional object of the present invention to perform the above objects without entraining and carrying-over solids from the sludge to downstream equipment.
It is an additional object of the present invention to provide a reliable apparatus to convey sludge to the coking zone while minimizing pluggage.
It is an additional object of the present invention to perform the above objects in a delayed coking process or in the fluid coking process. Preferably, the above process is performed in a delayed coking process.